Oscillator-modulator circuit



March 18, 1941.

K. EISELE 51' AL 0SCILLATOR-M0DUI.|A'1OR CIRCUIT Filed March 9, 1940 A W MQKEQQY MEENBQ v Patented Mar. 18, 1941 UNITED STATES PATENT OFFICE OSCILLATOR-MODULATOR CIRCUIT Germany Application March 9,

1940, Serial No. 323,090

In Germany October 31, 1938 2 Claims.

The invention is concerned with the solution of the problem of increasing or extending the frequency limits of the short-wave-band or bands in a superheterodyne receiver without the provision 5 of wave-band switch means and without necessitating an alteration in the relations between the minimum and the maximum capacities of the tuning circuits. It has been ascertained that the frequency limits of the short-wave ranges or hands are governed by the oscillation or tuning circuits of the local oscillator, while in the case of other bands it is the R. F. input circuits which impose limitations, if, as is usually true, the oscillator frequencies are higher than the incoming or signal frequencies. Research has resulted in this surprising discovery that the frequency limits of short-wave bands may be extended if the feedback or tickler coil of the oscillator is made of such limited dimensions that the maximum mixer slope of the mixer tube has not by far been reached. However, it would not do to choose such dimensions for the tickler coil for this would unduly impair sensitivity of the receiver.

The explanation of the action as stated is that when the :tickler coil is too large the natural frequency of the tickler coil comes to lie so close to the frequency range of the oscillator that the tuning circuit of the latter is detuned. Such detuning, in fact, has the same efiect as if the initial capacity of the circuit is raised. The natural frequency of the tickler coil when too close to the frequency band of the oscillator showed this further result that in assembly slight variations in the relative spacings of the wiring of the oscillator coil system occasionally caused wide variations in the natural frequency of the tickler coil and this means an impairment in the accuracy of the scale readings.

No such difficulties have been encountered inside the broadcast and the long-wave bands because in these wave-bands the resonance resistance of. the tuning or oscillatory circuit of the oscillator is higher so that a tickler coil which is small in comparison with the coils of the oscillator tuning circuit will serve the purpose. Whereas, for instance, inside the broadcast wave band the coil of the oscillation circuit comprises '70 turns and the tickler coil 25 turns, it has been necessary, inside the lowest short-wave band, to

use for both coils around 7 turns, and these were directly wrapped on top of each other. These difficulties are also absent in the short-wave range if the oscillation or tuning circuit of the oscillator is connected with the grid of the oscillator tube, 5 for this also is conducive to a reduced number of tickler coil turns. However, in the case of shortwave reception it is more favorable to unite the tuning oscillation circuit of the oscillator with the plate if the mixer tube is to be under the control of the automatic volume control system of the receiver. In volume control, as will be noted, the grid-filament capacitance undergoes a change owing to a change in the space-charge with the result that this would cause undue detuning of an oscillation circuit of the oscillator predominantly at the lower end of the wave band; that is, in the presence of lower tuning capacity. This is a fact which proves particularly unfavorable for short waves for the reason that small capacity changes occasion great frequency changes. Moreover, if the tuning circuit of the oscillator is associated with the plate of the oscillator the amount of current taken by the plate is reduced.

It will :thus be seen that the limitation imposed upon the short-wave range or band had to be tolerated if other drawbacks were to be avoided. Now, this difliculty is obviated by the present invention.

The invention is predicated upon the oscillator circuit organization known in the prior art in which the tuning circuit is included in the plate circuit of the oscillator tube and is in coupling relationship with the tickler coil contained in the grid circuit of the oscillator, and in which the said tuning circuit includes a padding condenser in series with the tuning-circuit coil, said condenser being dlmensioned so as to insure synchronism or tracking with the signal input circuit of the mixer or converter tube. According to the invention the said condenser at the same time is included in the grid circuit of the oscillator in series with the tickler coil; furthermore, the plate potential of the oscillator is so applied that the tuning circuit of the oscillator is practically not damped; and, finally, that the number of turns of the tickler coil is chosen just so great that the oscillator amplitude required for maximum attainable mixer slope will not be essentially exceeded at the point of the frequency band where the smallest oscillator amplitude prevails, or, more precisely, is just attained. If these dimensions are chosen it is feasible to cut down the number of turns of the tickler coil, within the short-wave bands, to around one-half with a resultant appreciable spreading of these bands as shall be demonstrated further below by concrete numerical examples.

By reference to the appended drawing wherein Fig. 1 is a circuit according to the invention, Fig. 2 is an explanatory diagram, and Fig. 3 shows the circuit for utilizing 3 bands, the invention shall now be described in greater detail.

Referring to Fig. 1, it Will be seen that the signal control grid of the mixer tube T is subject to regulator action produced by a control potential which is impressed by way of the resistance W1 and the input or signal oscillation circuit, with a View to insuring volume control. The rotary condenser C1 of the input circuit is ganged with the rotary condenser C2 of the oscillator tuning circuit as is usual. In order to obtain tracking or synchronism, as also known in the art, a shortening condenser is provided as indicated at C. The contour of the plates of rotary condenser Cz may thus be chosen equal to, or practically equal to that of the rotary condenser C1. The coil S of the oscillator tuning circuit is in coupling relationship with the ticker coil R. According to one object of the invention, the bottom end of the tickler coil R is not grounded, but is connectedwiththe point where coil S and the condenser C are joined. Thus the shortening condenser C is included also in the feedback circuit. Inasmuch as the phase of the potential arising at the condenser C is the same as that of the potential at the tickler coil, there occurs an increase in the regeneration or feedback, that is, more particularly at the upper limit of the wave-band when the rotary condenser is turned in in interleaved relation. Fundamentally speaking, by a reinforcement of the feedback at the upper end of the wave-band the problem underlying the invention would not yet be solved, if it were not for the fact that the feedback effect due to the tickler coil experiences a decline just at the upper limit of the wave-band. This is demonstrated in Fig. 2 where the wave-length A (abscissa) is plotted against the oscillator amplitude A (ordinate). Graph a illustrates the drop of the oscillator amplitude occurring in the absence of the invention, at the upper end of the waveband, the assumption being made that the oscillator tuning circuit is not subject to damping owing to the anode current feed (say, supply by way of W2) The droop in the graph a resides in the circumstance that the resonance resistance of the oscillator tuning circuit decreases at the top end of the wave-band. If the invention is carried into practice, as indicated by graph 19, the feedback effect at the upper end of the waveband, as contrasted with graph (1, is raised, for the reason that with growth of capacity C2 the voltage distribution between C2 and C is altered in such a wa that the potential rises at the condenser C.

However, this by itself would not yet solve the problem above outlined; in fact, to this end the number of turns of the tickler coil is reduced to a point where graph is obtained. For graph 11 is placed so high at the right-hand end the oscillator amplitude is already so great that the maximum mixer slope is approximately attained. In other words, raising the right-hand end of graph a in itself would neither result in increased gain nor any other advantage.

The second object of the invention above referred to, namely, to supply the plate potential to the oscillator in such a way that the oscillator tuning circuit will not be damped as a result to any practical degree must also be fulfilled if the problem above laid down is to be solved. It will be remembered that according to prior practice the plate potential is mostly fed by way of resistance W (shown dotted). This resistance W had a size of around 3G,000 ohms and it introduced damping of the oscillation circuit to a substantial degree at the lower end of the waveband where the resonance resistance of the oscillation circuit is high. At the upper end of the wave-band the resonance resistance has already dropped to an extent so that the damping additionally introduced by the resistance W will not cut any figure to speak of. Thus, by the use of a feedback only with the coil B there was obtained the graph 0 which is obtained also by the invention, though with a smaller number of feedback turns, provided the tickler coil is connected as stated and if at the same time the plate potential is app-lied by way of resistance W2.

If while using the circuit organization as shown in the drawing for the tickler coil, the plate potential were fed-by way of resistance W, then in lieu of graph b a curve would result exhibiting a decline from the right to the left-hand side, for the reason above pointed out that resistance W" produces more damping at the lower end of the wave-band. However, this would mean no advance as the feedback turns are governed by the lowest point of the curve if the slope or mutual conductance obtainable with the mixer tube is to be utilized.

Where the ways and means here disclosed are used in practice, it would also be possible to use an RF choke-coil. in lieu of the resistance W2, for the point is that the RF potential prevailing at the condenser C is not short-circuited. However, an ohmic resistance is less costly, while at the same time offering the advantage that the plate potential of the oscillator is reduced to the desired value. The invention and its underlying idea Would, in fact, be carried into practice also if instead of resistance W a choke-coil were included in the circuit organization inasmuch as a chokecoil introduces no damping in the oscillation circult. However, this circuit arrangement also is more expensive than the use of a resistance W2 of, say, 25,000 ohms.

In order to make the advantage residing in the invention still more evident a number of numerical examples shall here be cited. Receivers as heretofore designed cover the following wavebands numbered I to HI:

I 13. 7- 43 m. (21,9007,000 kc.) II 40 130 m. 7, 500-2, 300 kc.) III 182 -561 m. 1, 565- 535 kc.)

Where the basic idea of the invention is used, these wave-bands were secured with the same receiver apparatus:

I 13. 7- QB in. (21, 900-6, 120 kc.) II 47 -176.4= m. 6,400-1,700 kc.) III 176.4-572 m. 1, 700- 525 kc.)

III all three bands come to adjoin closely. In other words, what is gained are the waves from to around meters.

The necessary numbers of turns of the oscillator, in the old receiver set were as follows:

Number of turns 'lickler Tuning circoil cuit coil In contrast, the numbers of turns required in the receiver designed along the lines of the in- Vention are as follows:

Number of turns Tickle: Tuning circoil cuit coil It will be noticed that for band I the tickler coil was cut down from 6 turns to 3 /2 turns. In band II the number of required tickler coil turns was also cut in half, from 9 to 4 The number of turns of the tuning circuit coil however was required to be increased from 26 to 33 turns since for band II the lower limit of the range had been advanced from 40 m. to 47 m.

For bands I and II the oscillator coils were of single layer and Were wound on separate cylindrical forms with the coils R and S for each band placed end to end. .For band III multilayer cross-wound coils were used.

In order to accommodate the tickler coil and the tuning circuit coil having the greater number of turns on the same size form, the tickler coil was made of thinner wire, that is, 0.3 instead of 0.5 mm. in diameter.

In Fig. 3 is shown the circuit organization used for covering the three said wave-bands. Inside range I only the tuning circuit coil I is active, for switches S2, S3, and S4 are closed. Thus, only the series padding condenser C3 is operative. The middle tickler coil could be short-circuited, though this is not absolutely necessary since the invention allows such a marked reduction in size of the tickler coil that the said middle tickler coil, in range I, will not cause the natural frequency of the feedback coil to come too close to the band.

In range II coils I and 2 are active for then the switch S2 is opened and the switches S3 and S4 are closed. In that case the condensers C2 and C4 in series are connected in circuit and act .densers C3, C4 and C5 operate as padding condensers. For feed-back purposes only condensers Ca and C4 are relied upon. The switch S5 which is indicated by the dash-line will be required and be closed in band III only if there is a risk of the upper coils being excited on a short Wave.

What we claim is:

1. In a superheterodyne receiver for the reception of short waves, a local oscillator circuit adapted to generate a frequency of substantially constant voltage over a comparatively wide range of frequencies for heterodyning with the received signal frequency to produce an intermediate frequency, said local oscillator utilizing a tube having at least a cathode, grid and anode, a tunable oscillation circuit connected between the anode and cathode, said oscillation circuit comprising a coil and a fixed condenser connected in series therewith and a variable condenser in shunt across the coil and fixed condenser, a feed-back coil having substantially half the number of turns of the first coil in coupling relation to the latter coil eifectively connected between the grid and the junction of the first coil and fixed condenser, and a connection between said junction and a source of anode potential.

2. In a superheterodyne receiver for the reception of short waves in a plurality of different bands, a local oscillator circuit adapted to generate a frequency of substantially constant voltage throughout the range of said bands for heterodyning with the received signal frequency to produce an intermediate frequency, said local oscillator utilizing a tube having at least a cathode, grid and anode, a tunable oscillation circuit connected between the anode and cathode, said oscillation circuit comprising a plurality of coils and an equal number of fixed condensers all connected in series and a variable condenser in shunt across said coils and fixed. condensers, an equal number of feed-back coils each in coupling relation to a corresponding coil of the first mentioned coils, switch means for short-circuiting certain of the coils and fixed condensers to select a diiierent operating band, at least one of the fixed condensers remaining in circuit serving as a feed-back condenser and another as a tracking condenser, and a connection from a source of positive potential to the junction between one of the plate coils and its adjacent fixed condenser.

HANS FAHRNSCHON. KARL EISELE. 

